Heat-resistant electric insulation and method of manufacture



i 0 6 '8 4 CROSS REFERENCE Sept. 29, 1959 K H ETAL 2,906,649

HEAT-RESISTANT ELECTRIC INSULATION AND METHOD OF MANUFACTURE Filed April 6, 1956 United States Patent HEAT-RESISTANT ELECTRIC INSULATION AND IVIETHOD OF MANUFACTURE Heinz Keuth, Altenfurt, near Nurnberg, and Hans-Werner Rotter and Paul Guillery, Numberg, Germany, assignors to Siemens-Schuckertwerke Aktiengesellschaft, Berlin-Siemensstadt, Germany, a corporation of Germany Application April 6, 1956, Serial No. 592,703 Claims priority, application Germany April 7, 1955 21 Claims. (Cl. 117-230) Our invention relates to the production of electrically insulating and heat-resistant coatings and films of inorganic material and, in one of its more particular aspects, to thin inorganic coatings which dagmrlly to metal-sheet material so as to permit ben ing and machini g e materia without impairing the insulating coating. More specifically, the invention also relates to insulationcoated magnetic steel sheets for the core stacks of transformers, reactors, generators, motors or other electromagnetic apparatus, and to dielectric foils or coatings for capacitor electrodes.

It is among the objects of our invention to provide an insulating layer or coating which can be readily applied to cold metal sheets, which will dry rapidly, particularly while passing through an oven or dryer in continuous operation, and which immediately solidifies to great strength and hardness. The insulating layer or coating must also have a high insulating quality and homogeneity at extremely small thickness, preferably below 10 microns, to secure good space utilization when stacking such insulated sheets of magnet steel.

It is another object to make such insulating coatings capable of resisting mechanical stresses, such as bending and punching, without peeling or scaling off. The coatings are further supposed to be scratch-resistant to prevent being damaged by handling and stacking the coated metal sheets.

Another object is to produce such inorganic insulating coatings which do not scale or peel olf the metal sheets when annealing the sheets in a reducing atmosphere, as is needed for certain magnetic sheet materials. The coated sheets in a stack do not stick together when annealing the stack; and the above-mentioned mechanical and electrical properties remain preserved after annealing. Furthermore, the coatings are oiland water-resistant before and after annealing.

These various objects and desiderata, to our knowledge, have never been conjointly obtaining by the known mica or asbestos coatings heretofore available. However, we have found that many or all of the desired qualities are achieved by proceeding, according to our invention, as described presently.

The raw material for producing our coatings is silica mineral substance generall he mica grou such as mica proper and mica-like substances such as asbestos,

bentonite and other silica minerals, all being aluminumcontaining silicates and having in common that ey con- .ta watefin of crystallization.

Included in t e group of applicable mica-family substances are the synthetic micas such as fluorine mica. Further appertaining to this group are muscovite (potassium mica), phlogopite or biotite (magnesium micas), lepidolite or zinnwaldite (lithium micas), roscoelite (vanadium mica); also oellacherite, fuchsite, mariposite, paragonite, pholidolite, anomite, lepidomelan, manganophyll, ganophyllite, rubellite (rubellan), culsageite, hal- 2,906,649 Patented Sept. 29, 1959 ICC lerite, philadelphite, polylithionite, taeniolite, margarite,

ephesite and others.

he mica-like or near-mica substances applicable for t the purposes oi the invention are those minerals that 5% possess a crystallingflggctural relation to pyggphyllite, such as hydrgphyllite, and also the incomplete or altered 3 micas having stratified crystals that are valency-unsaturated and, in most cases, occur with a reduced potassium content and an increased content of water. Appertaining 1 to the latter kind of suitable substances are verrpigilite l and the hydromicas such as hydromuscovite, levernerite, lguembelite, hydrobiotite, Igkwuitgand seladonite; also; the illites which generica y comprise the mica-like clay; minerals, such as serizite, which in contrast to clay proper} tdo not exhibit intra-crystalline swelling. 1

, Applicable synthetic micas, aside from fluorine mica,

fare the hydrothermal micas including those in the build- -ing-up stage as well as the completely formed crystals.

Using any one or more of such combined water-containing minerals of the mica family as a raw material, we

proceed as follows.

The mineral substance is first subjected to heating at a temperature above 100 C. and for a period of time suflicient to expel part of the water of crystallization,

preferably a major portion of the combined water but not all of it.

Thereafter the substance is mechanically reduced within a liquid consisting at least partly, but preferably entirely, of water. The comminution is effected, for instance, by a means of a rapidly rotating hammer mill. Then the particles smaller than one micron are segregated from the resulting product and employed to make the coating. A preferred way of doing this is to permit the product to settle until only particles below the size limit remain in suspension within the top zone of the liquid,

whereafter the top zone is decanted. An aqueous dispersion of the desired consistency is then formed of the separated particles, that is, the decanted particles, and the dispersion is deposited onto the metal sheet or other body to be coated. This is done by spreading the dispersion like a varnish or paint with a brush, by spraying,

by immersing the body into the dispersion, by electrophoresis or in any other suitable manner. Thereafter the coating is dried. The individual mineral particles then adhere firmly to one another and to the surface of the coated body.

Particularly favorable coatings are obtained if the raw mica is first calcined by heating but only to such an extent that it still retains part of its water of crystalliz'ation. During calcination, the mica disintegrated into scales. After further mechanical reduction in water, the resulting product is permitted to settle forsuch a period of time that only particles of an average size of 0.5 micron remain in supsension. If one plots thEYfEEEEiiS" mirrence of particles of a given size versus the particle size to obtain a curve, then this curve steeply declines from higher to lower values when the particle size of one micron is exceeded, whereas the slope of the curve is much less toward the smaller particle sizes.

Mainly for the manufacture of annealing-resistant coatings on sheet metal, it is preferable to add to the dispersion of the small mica particles an inorganic colloid, particularly water glass (sod' silicate). Optimum conitions result" a out 20% 11mm water glass of I ommercial dilution, i.e. of the gravity 1.34, is added to 65 be water.

r When no water glass or similar colloid is added so that the finished coating consists of mica particles alone, a smooth coating resistant to incandescent temperature is obtained, but this coating is not sufiiciently scratchresistant for some purposes. On the other hand, when a coating is formed of water glass alone or together with any of the heretofore customary fillers such as heavy spar gbarite), zinc oxide pg irpn oxide, then the coating cracks when sub ected 'to annealing and turns into a fine dust that can be wiped off. The mixture of both kinds of substances, however, results in a very hard coating which, when annealing the coated bodies, causes neither sticking of the sheets nor cracking or scaling of the coating. Particularly favorable results are obtained with a colloidal dispersion containing 20% by weight of ieaims and an addition of 10% by weight of water wwmrdaLdilmionni ravity 1.34). The a amoun water glass may be smaller or larger than However, when the water-glass content is too small (below 5%), the coatings are soft, whereas when the water-glass content is too large (above there i may occur a local formation of bubbles and a scaling of the insulating coating after annealing, depending upon the condition of the sheet surface and the annealing \conditions.

l, Due to the addition of water glass to the mica suspension, the somewhat swellable mica and the water glass react with each other in such a manner that a th ,thixotropiegelmgqdgged. With the above-mentioned composition of the mixture, this gel is nevertheless still sprayable because the stream of compressed air used for spraying destroys the gel structure. After the sprayed mixture is deposited on the sheet metal to be coated by insulation, the liquid again assumes the original gel character and produces a thin layer which does not run off and dries rapidly. Such coatings have the appearance of a thin, white and smooth varnish coating. They do not require any particular preparation of the sheet surface, can be applied with the facility of a varnish coating, and have excellent adhesion even on smooth sheet surfaces. Because of the slight thickness of the coatings, a good degree of filling or space utilization by magnetic material within a stack of coated sheets is achieved.

w gtinmditha.taaatstslasag Js inorganic co loids may al so consist of bentopitet qrswet ting :agents like Nekal, or b ifi di'ffiag'erits. Km smaller amount of added binding agent is sufiicient than in the dispersions heretofore used as an insulating varnish. A varnish or insulating paint poor in binding agents has so far become known only for graphite ground to colloidally fine grain size. Such a paint or varnish, however, serves different purposes, namely for producing a conductive coating, for instance on insulators, non-conductive articles that are to be galvanized or electroplated, and the like. As compared with the known mica dispersions which are processed in the manner of paper manufacture to form sheets of mica, the dispersions according to the present invention are distinguished by a much higher degree of comminution of the small particles. We have discovered that by comminuting the raw material to fines of a much smaller degreeof size than heretofore known, a dispersion is obtained which can be used and fabricated as readily and in the same manner as varnish and which afiords the production of exceedingly thin coatings.

The drawing shows schematically and on enlarged scale a coated magnetic steel sheet in Fig. 1 and an electric capacitor in Fig. 2.

According to Fig. 1, the steel sheet 1 is coated with an extremely thin layer 2 of mica fines made as described above. The capacitor shown in Fig. 2 is composed of electrodes, of which only three are shown at 3, 4, 5, and of intermediate dielectric coatings 6, 7, 8 of mica fines made as described below.

Example 1 Muscovite is calcined at 780 C. for approximately one hour. Subsequently the mica, still in hot condition is placed into distilled water with a pH value below 6.

The same results are obtained when the mica is permitted to cool before immersing it in water.

Within the water, the mica is reduced by means of a hammer mill rotating at approximately 12,000 rotations per minute. Subsequently the comminuted product is permitted to settle in the water. The coarser particles then sink to the bottom. Particles of smaller grain size remain in suspension within the lower portion of the vessel. In the upper portion of the vessel near the level of the water bath, a pulp is formed consisting of a suspension of rnlgg particles smaller than one micrgn. This pulp is decanted. e remainder of the pulp 'can again be subjected to a reducing and settling process as described above, and each time the upper pulp zone containing the mica fines below one micron can be decanted. The decanted pulp contains a large amount of water. It is thickened by evaporating a sufficient amount of water to assume the desired consistency, for instance that of a paint or varnish. The thickened suspension is then applied to the surface of metal sheets in the manner of a paint or varnish as described in the foregoing. The varnish-like coating is permitted to dry. Thereafter the coating remains firmly adhering even if the metal sheets are immersed in water.

Example 2 glass and about 70% water. WWO After rymg S n e u I e o the coating, the sheet metal was annealed at 600 to 800 C. The resulting coating was resistant to scratching, and did not scale off when subjecting the sheet to bending and punching.

Example 3 The thickened dispersion produced in accordance with Example 1 and having a pH value of approximately 5 is placed into a vat. A metal sheet to be coated was immersed in the vat and was connected to one pole of a voltage source. An electrode was placed opposite the sheet to be coated. The electrode had a shape and size similar to the sheet and was connected with the other pole. Current was passed through the dispersion between sheet and electrode to electrophoretically precipitate the mica fines upon the sheet. The result was a dense white layer whose thickness is dependent upon the time period of the current supply and the applied voltage as well as upon the electric resistance of the dispersion.

In the manner exemplified by Example 3, coatings of any desired small thickness between 3 to 10 microns, as well as considerably thicker layers, can be produced. Particularly with very thin coatings, the precipitation, after drying, exhibits the properties of a smooth varnish coating which can be heated to glowing temperatures without scaling off.

When the body to be coated, for instance a sheet or tape of metal, is drawn through the electrolyte, such coatings can be produced in a continuous method.

If the precipitated coating is given suflicient thickness and if copper is used as the body or electrode upon which the mica particles are precipitated, then, after drying, the coating can be peeled off as an independent, firmly coherent layer. In this manner, thin heat-resistant insulating foils are produced as are often required for electrical apparatus. For instance, such foils are applicable as a dielectric for capacitors. However, the dielectric layer may also be precipitated directly upon the capacitor electrode in the manner described in the foregoing. Electrode tapes for capacitors, alternating with the abovementioned foils, can be wound up to form capacitors.

The above-mentioned electrodes directly coated with mica can likewise be given the form of a tape and can be wound up to form capacitors without impairment of the insulating coating.

Another way of producing electric capacitors according to the invention is as follows: At first a capacitor electrode (3 in Fig. 2) is provided with a mica coating 7 as described above. Any necessary terminal means such as conductive strips of foil may then be added. Thereafter another electrode layer 4 is produced on the insulating coating by spraying metal onto the coating. The metal layer is then again coated with insulating material of mica fines 8 according to the invention, and so forth.

Particularly suitable are coatings according to the invention, especially coatings of extreme thinness, for magnetic sheets with preferred magnetic orientation, which in one direction have a considerably higher magnetic permeability than in the direction perpendicular thereto. The good magnetic properties of such sheets are greatly impaired by mechanical fabrication, for instance when the sheets are punched or cut. These properties, however, can be restored by subsequent annealing. The insulating coatings according to the invention, as mentioned, readily withstand the machining or other fabrication of the sheets as well as the annealing. The sheets can be annealed in stacks because, as likewise mentioned, the insulating coatings according to the invention prevent the sheets from sticking together.

We claim:

1. The method of producing upon a metallic surface structure a thin varnish-like, heat-resistant and electrically insulating, inorganic coating having good coherence without an additional binding agent, comprising the steps of partially calcining a mica-group mineral by heating to a temperature and for a length of time sufficient to expel only a part, but a major part, of its water of crystallization, mechanically comminuting said mineral within an aqueous liquid which is weakly acid with a pH of approximately 5 to 6, separating from the comminuted product by decanting the particles smaller than one micron in said weakly acid aqueous liquid, forming an aqueous dispersion containing said liquid and said particles, covering a metal surface structure with a layer of said dispersion, and drying the layer to produce the heat-resistant coating.

2. The method according to claim 1, wherein the liquid of said aqueous dispersion consists of approximately 80% water and about 20% water glass of commercial dilution.

3. The method of producing a thin, varnish-like, heatresistant, electrically insulating inorganic coating, comprising the steps of heating a crystalline silica mineral of the mica group to expel only part, but a major part, of its water of crystallization, at not higher than about 780 C., mechanically reducing the mineral within aqueous liquid which is weakly acid with a pH of approximately 5 to 6, permitting the resulting product to settle until the particles remaining in dispersion are smaller than one micron, decanting the dispersion, covering a metal surface structure with a layer of the dispersion, and drying the layer to produce the heat-resistant coating.

4. The method of producing a thin, varnish-like, heatresistant, electrically insulating inorganic coating, comprising the steps of partially calcining by heating a micagroup mineral to expel only part of its water of crystallization, mechanically reducing the mineral within aqueous liquid which is weakly acid with a pH of approximately 5 to 6, permitting the resulting product to settle in the liquid until the particles remaining in dispersion have an average size of 0.5 micron, decanting the dispersion, separating part of the water from the dispersion to obtain a material of varnish-like consistency, covering a metal surface structure with the material to produce thereon the heat-resistant coating upon drying of the material, the amount of dispersion applied to the metal surface being limited to that required to form a layer between about three to ten microns thick, the method being carried out at temperatures not higher than about 800 C.

5. The method of producing a heat-resistant coating on a metallic surface structure, comprising the steps of incompletely calcining a swellable mica-group mineral to expel only a portion of its water of crystallization, mechanically reducing the mineral, until at least a portion thereof is under one micron, within an aqueous liquid which is weakly acid with a pH of approximately 5 to 6, separating from the reduction product a dispersion of said weakly acid liquid with particles of the incompletely calcined mineral which are no larger than one micron, adding an inorganic colloid to the aqueous dispersion to form with said dispersion a thixotropic gel material of varnish-like consistency, applying said thixotropic gel material to a surface structure to form a layer thereon, whereby during said applying step said thixotropic material loses the character of a gel and converts to a sol, said material again assuming the character of a gel after it is deposited on said surface structure.

6. The method according to claim 5, wherein said inorganic colloid is water glass.

7. The method of producing a thin, varnish-like, heatresistant, electrically insulating inorganic coating on metal, comprising the steps of expelling only part of the combined water from muscovite, comminuting the muscovite within aqueous liquid which is weakly acid with a pH of approximately 5 to 6, separating from the reduction product the particles smaller than one micron, forminga weakly acid aqueous dispersion of the separated particles, coating the metal with the dispersion, drying the coating on the metal and subjecting it to annealing temperature.

8. The method of producing a heat-resistant coating on a surface structure, comprising the steps of incompletely calcining a swellable mica-group mineral to expel only a portion of its water of crystallization, mechanically reducing the mineral, until at least a portion thereof is under one micron, within an aqueous liquid which is weakly acid with a pH of approximately 5 to 6, separating from the reduction product a dispersion of said weakly acid liquid with particles of the incompletely calcined mineral which are no larger than one micron, removing part of the water from the dispersion, adding water glass in an amount smaller than the remaining water to form with said dispersion a thixotropic gel material, applying said thixotropic gel'material to a surface structure to form a coating layer thereon, whereby during said applying step said thixotropic material loses the character of a gel and converts to a sol, said material again assuming the character of a gel after it is deposited on said surface structure, drying said coating layer, and heating the coated surface structure to a temperature between about 600 and 800 C.

9. The method of producing upon a metallic surface structure a thin varnish-like, heat-resistant and electrically insulating, inorganic coating having good coherence without an additional binding agent, comprising the steps of partially calcining a mica-group mineral by heating to a temperature and for a length of time sufficient to expel only a part, but a major part, of its water of crystallization, mechanically comminuting said mineral within an aqueous liquid which is weakly acid with a pH of approximately 5 to 6, separating from the comminuted product by decanting the particles smaller than one micron in said weakly acid aqueous liquid, forming an aqueous dispersion containing said liquid and said particles, immersing in the dispersion a surface structure to be coated, depositing on said surface structure by electrophoresis a coating from said dispersion, and drying the coating on the surface structure.

10. The method of producing on dynamo steel sheets an electrically insulating thin coating resistant to incandescent temperature, comprising the steps of partially calcining by heating mica to liberate only part of its combined water at not higher than about 780 C., mechanically reducing the mica in water which is weakly acid with a pH of approximately to 6, permitting the comminution product to settle until only fines of less than one micron grain size remain in dispersion, decanting the dispersion, evaporating part of the water, adding water glass in an amount less than the remaining water, coating the resulting dispersion product onto the dynamo steel sheets, drying the coatings on the sheets, and subjecting the coated sheets to annealing at a temperature between about 600 and about 800 C., the amount of dispersion applied to the metal surface being limited to that required to form a layer between about three to ten microns thick.

11. The method of dielectrically coating an electrode tape for electric capacitors, which comprises the steps of partially calcining by heating mica to expel only part of its combined water, mechanically reducing the mica in water which is weakly acid with a pH of approximately 5 to 6, permitting the comminution product to settle until only fines of less than one micron grain size remain in dispersion, decanting the dispersion, evaporating part of the Water therefrom, passing the electrode tape through the dispersion and applying electric potential between the tape and an electrode to precipitate by electrophoresis a coating onto the tape, and drying the coating.

12. The method of producing inorganic electrically insulating heat-resistant foils, which comprises the steps of partially calcining by heating a mica mineral to expel only part of its combined water, mechanically reducing the mineral within aqueous liquid which is weakly acid with a pH of approximately 5 to 6, separating from the reduction product the particles smaller than one micron, forming an aqueous dispersion of the separated particles, covering a surface sturcture with a layer of the dispersion, drying the layer, and peeling the layer oil? the surface structure.

13. The method according to claim 12, wherein surface structure consists of copper.

14. In combination, a metal body and a heat-resistant varnish-like insulating coating on said body, said coating consisting substantially of dispersed particles of mica all smaller than one micron, said particles retaining part of their original water of crystallization, and said coating having a thickness of about three to about ten microns.

15. In combination, a metal body and a heat-resistant insulating coating on said body, said coating consisting substantially of dispersed particles of mica all smaller than one micron in mixture with water glass, said particles retaining part of their original water of crystallization, said coating being about three to about ten microns thick.

16. A magnetic steel sheet having an insulating coating consisting of a thixotropic dispersion of mica particles of less than one micron particle size in water glass, said layer being about three to about ten microns thick.

the

17. An electric capacitor having a tape electrode coated with a thixotropic layer of a mixture of mica particles and water glass, said particles having all a size below one micron, and said layer being about three to about ten microns thick.

18. The method of electrically insulating metal sheets comprising applying thereto an aqueous dispersion which is weakly acid with a pH of approximately 5 to 6 of finely divided particles of a partially calcined mica mineral, the particles having an average size of 0.5 micron and part of their original water of crystallization, the dispersion being thixotropic and containing between 5 and 20 percent by weight of water glass, and thereafter drying and annealing the coated sheet.

19. The method of electrically insulating metal sheets comprising applying thereto a coating comprising an aqueous dispersion which is weakly acid with a pH of approximately 5 to 6 of finely divided particles of a calcined mica mineral having an average size of 0.5 micron and part of their original water of crystallization, the particles having a maximum size of not more than one micron, the dispersion being thixotropic and containing between 5 and 20 percent by weight of water glass, and thereafter drying the coated sheets, the amount of dispersion applied being limited to that required to form a layer between about three to ten microns thick.

20. The method of electrically insulating metal sheets comprising applying thereto a thin coating comprising an aqueous dispersion which is weakly acid with a pH of approximately 5 to 6 of finely divided particles of a partially calcined mica mineral having an average size of 0.5 micron and part of their original water of crystallization, the particles having a maximum size of not more than one micron, the dispersion containing sufiicient water glass to produce a thixotropic liquid, the amount of dispersion applied being limited to that required to form a layer between about three to ten microns thick, and thereafter drying the coated sheet.

21. The method of claim 20 in which the mineral is mica.

References Cited in the file of this patent UNITED STATES PATENTS 686,930 Heard et a1 Nov. 19, 1901 1,310,939 Bancroft July 22, 1919 1,578,813 Dawes et a1 Mar. 30, 1926 1,867,362 Lathrop July 12, 1932 1,963,276 Miner et al. June 19, 1934 2,076,898 Labus et a1 Apr. 13, 1937 2,081,935 Jones June 1, 1937 2,215,295 Morrill Sept. 17, 1940 2,745,141 Brennan May 15, 1956 FOREIGN PATENTS 738,357 Great Britain Oct. 12, 1955 

1. THE METHOD OF PRODUCING UPON A METALLIC SURFACE STRUCTURE A THIN VARNISH-LIKE, HEAT-RESISTANT AND ELECTRICALLY INSULATING, INORGANIC COATING HAVING GOOD COHERNECE WITHOUT AN ADDITIONAL BINDING AGENT, COMPRISING THE STEPS OF PARTIALLY CALCINING A MICA-GROUP MINERAL BY HEATING TO A TEMPERATURE AND FOR A LENGTH OF TIME SUFFICIENT TO EXPEL ONLY A PART, BUT A MAJOR PART, OF ITS WATER OF CRYSTALLIZATION, MECHANICALLY COMMINUTING SAID MINERAL WITHIN AN AQUEOUS LIQUID WHICH IS WEAKLY ACID WITH A PH OF AP- 